September 11, 2012

Study: Forest ‘Greenness’, Health Linked To Winter Snowfall

The forests that skirt high peaks have always been dependent on the accumulation of winter snowfalls that subsequently melt and provide valuable moisture down the steep mountain slopes.

A team of scientists, led by University of Colorado-Boulder scientists Ernesto Trujillo and Noah Molotch, has investigated how the trends of rising temperatures and declining water availability have influenced these mountain forests over the past 50 years. Their report is featured in Nature Geoscience.

According to their report, as the warming in spring and summer starts earlier and earlier–the resulting rapid and heavy snowmelt is becoming associated with less greenery, more wildfire activity, and greater tree mortality rates in mountain forests across the western United States.

To measure the amount of ℠greenness´ in these areas, the team used satellite images and ground measurements to first locate where mid-level forests sustained by moisture transitions into higher-elevation forests that depend more on sunlight.

"The research demonstrates yet another complexity in the response of mountain ecosystems to increasing temperatures," said hydrologist Tom Torgersen, program director at the National Science Foundation, which funded the research. "High-elevation mountain forests are typically temperature-stressed and low-elevation mountain forests are often water-stressed.”

"At mid-elevations, 'everything is just right'--until it goes wrong," Torgersen explained. "Higher temperatures lead to reduced snowpack and reduced water availability, leaving trees at mid-elevations more stressed and more prone to fires."

In the study, the team used 26 years of continuous data from the Advanced Very High Resolution Radiometer located onboard a National Oceanic and Atmospheric Administration satellite, to measure the forest greenness. These images were compared with historical data from 117 snow stations maintained by a collaboration of state and federal agencies.

Additional information was gathered from "flux towers" located in the Sierra Nevada mountain range. These towers are used to measure the exchanges of carbon dioxide, water vapor and energy between the land and the atmosphere. According to a statement from the NSF, the data from these towers matched up well with the satellite and ground data.

"These results provide the first direct observations of snowpack-forest connections across broad scales," said Molotch. "The strength of the relationship between forest greenness and snowpack from the previous year is very surprising.”

According to the report, about 50% of the variability in greenness seen by satellites in mid-elevation forests is connected with maximum snowfall accumulation from the previous winter. The other 50% of variability is related to other factors, such as soil depth, soil nutrients, and sunlight levels.

The research noted that climate change trends portend decreasing levels of healthy green forests at higher elevations. They said future research should monitor where the ℠tipping point´ of mid-level forests are located to predict the potential for forest fires or other tree killing factors.

"Finding the tipping point between water-limited [mid-elevation] forests and energy-limited [high-elevation] forests defines the region of the greatest sensitivity to climate change--the mid-elevation forests--which is where we should focus future research,” Molotch said.

"If we look ahead to the time when climate models are calling for warming and drying conditions, the implication is that forests will be increasingly water-stressed in the future and more vulnerable to fires and insect outbreaks."